# std::optional<T>::or_else

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< cpp‎ | utility‎ | optional

template< class F > constexpr optional or_else( F&& f ) const&; | (1) | (since C++23)
template< class F > constexpr optional or_else( F&& f ) &&; | (2) | (since C++23)

Returns *this if it contains a value. Otherwise, returns the result of f.

The program is ill-formed if std::remove_cvref_t<std::invoke_result_t<F>> is not same as std::optional<T>.

1) Equivalent to return *this ? *this : std::forward<F>(f)();. This overload participates in overload resolution only if both std::copy_constructible<T> and std::invocable<F> are modeled.
2) Equivalent to return *this ? std::move(*this) : std::forward<F>(f)();. This overload participates in overload resolution only if both std::move_constructible<T> and std::invocable<F> are modeled.

### Parameters

f | - | a function or Callable object that returns an std::optional<T>

### Return value

*this or the result of f, as described above.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_optional` | `202110L` | (C++23) | Monadic operations in std::optional

### Example

Run this code

    #include <iostream>
    #include <optional>
    #include <string>

    int main()
    {
        using maybe_int = std::optional<int>;

        auto valueless = []
        {
            std::cout << "Valueless: ";
            return maybe_int{0};
        };

        maybe_int x;
        std::cout << x.or_else(valueless).value() << '\n';

        x = 42;
        std::cout << "Has value: ";
        std::cout << x.or_else(valueless).value() << '\n';

        x.reset();
        std::cout << x.or_else(valueless).value() << '\n';
    }

Output:

    Valueless: 0
    Has value: 42
    Valueless: 0

### See also

value_or | returns the contained value if available, another value otherwise (public member function)
and_then (C++23) | returns the result of the given function on the contained value if it exists, or an empty `optional` otherwise (public member function)
transform (C++23) | returns an `optional` containing the transformed contained value if it exists, or an empty `optional` otherwise (public member function)

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